1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to an apparatus and a method for estimating and compensating an initial carrier frequency offset in a receiver of a wireless communication system according to an orthogonal frequency division multiple access (OFDMA).
2. Description of Related Art
Much research in various fields to realize the fourth generation (4G) mobile communications is in progress. In 4G mobile communication according to IEEE 802.16d/e, WiBro, WiMAX, etc., a wireless LAN network, a digital audio and video broadcasting network, etc., as well as a satellite communication network is integrated into single network to systematically work together. Accordingly, service for a user may be smooth and optimal in the 4G mobile communication network.
A mobile device (Portable Subscriber Station) may communicate with a base station (Radio Access Station) by processing a preamble signal received from the base station system to synchronize a system. A downlink (DL) preamble signal is transmitted from the base station system at the first symbol location every frame. Accordingly, the mobile device synchronizes a system based on the preamble signal, searches a cell, and demodulates data following the preamble signal transmitted from the base station system.
FIG. 1 is a block diagram illustrating a conventional wireless communication system 100. In the conventional wireless communication system 100, transmission data is modulated according to a predetermined method in a modulator 110 of a transmitter. The modulated data is loaded onto a carrier signal and transmitted through a channel via a transmitter (Tx) circuit 120. The signal which is transmitted through a channel is received in a receiver (Rx) circuit 150 of a receiver and is affected by additive white Gaussian noise (AWGN). The Rx circuit 150 converts the received signal into a baseband signal, and a demodulator 160 demodulates the baseband signal to acquire the transmission data.
In an OFDMA system, for instance, the demodulator 160 synchronizes the system according to the preamble signal from the first symbol location of the downlink (DL), searches for a cell and demodulates data following the preamble signal. A method for estimating an initial carrier frequency offset in a frequency domain may be utilized to synchronize a mobile device system and receive data transmitted from a base station system. However, the frequency offset estimation method in the frequency domain, which may estimate a fine frequency offset by using pilot symbols, can be used after an initial synchronization process due to unknown Fast Fourier Transform (FFT) timing in the frequency domain.
Accordingly, the initial carrier frequency offset estimation shall be performed in the time domain. That is, if there is the same frequency offset in repeated samples in the time domain, a corresponding offset can be estimated by using repetitive pattern characteristics of training symbols of the OFDMA preamble.
FIG. 2 is a block diagram illustrating a conventional carrier frequency offset estimator 200. In FIG. 2, when a received signal r(t) is sampled and stored in a register 210, repetitive signal samples in every Nth, for example, r(n) and r(n+N), r(n+1) and r(n+N+1), . . . , r(n+d) and r(n+N+d), etc., are processed in complex conjugators 211, 213, . . . , 215 and multipliers 212, 214, . . . , 216. An addition/average calculation unit 220 adds and averages outputs of the multipliers 212, 214, . . . , 216, thereby acquiring a result. A calculator 230 performs an arc tangent operation for the result from the addition/average calculation unit 220 and generates a phase offset value. Accordingly, a multiplier 240 acquires a frequency offset value Df by multiplying the phase offset value by −1/(2πN).
For example, in an OFDMA system according to IEEE 802.16d/e standard specifications, a transmitted/received signal, as shown in FIG. 3, has 3 repetitive periodic characteristics in the time domain according to preamble symbol of the first symbol location of the downlink (DL) after the CP (Cyclic Prefix) interval. In FIG. 4, the 1024 subcarriers of the preamble symbol in IEEE 802.16d/e specification have a configuration that 852 subcarriers, excluding guard intervals (GI) including 86 subcarriers, are separated into 3 segments and transmitted/received. Here, the 1024 subcarriers have repetitive periodic characteristics every 341.3 subcarriers. Accordingly, since the 1024 subcarriers are transmitted in one non-zero subcarrier with two zero subcarriers, when the 1024 subcarriers are sampled, the sampled subcarriers experience periodically phase offset with each other according to the segment numbers.
Accordingly, in the frequency offset estimator 200 according to the conventional art which calculates a frequency offset according to an averaging mechanism and a phase compensation method, as shown in FIG. 2, since the number of samples used in the averaging mechanism is not taken into consideration, the estimated frequency offset is not accurate. Also, the frequency offset estimator 200 requires many hardware and software resources such as the complex conjugate number operators 211, 213, . . . , 215, the multipliers 212, 214, . . . , 216, the addition/average calculation unit 220, etc., thereby lowering link performance. Accordingly, an apparatus which estimates an accurate frequency offset for each of the segment numbers and synchronizes the mobile device system with less resources shall be necessary.